Many people rely on earplugs to protect their hearing from loud noises. In many applications, these devices merely need to reduce noise to a safe level, but some users are more demanding: the attenuated sound level should be acoustically “flat” (i.e., the relative levels of different audible frequencies should be similar to the original levels, but reduced across the entire sound spectrum for safety and comfort). This requirement is especially common among musicians and music technicians, who may play or work in a dangerously loud environment, but who still must be able to hear and judge the tonality of the sound.
Among these users, there is often the added requirement that another signal be added to the attenuated ambient sound. For example, a timing or “click” track, a previously-recorded audio track, or an amplified version of the user's own instrument or voice might be played directly into the user's ear, using a system of headphone audio drivers.
Earphones, also known as In-Ear Monitors (“IEMs”) that fulfill these requirements are well-known. It is common for these to be crafted into a customized mold of the wearer's outer ear, and to extend somewhat into the ear canal. These devices seal well to the ear canal, so they are very effective to control and reduce the volume or amplitude of ambient sounds. Further, the portion that rests in the outer ear is large enough to contain one or more small headphone drivers, which can reproduce the additional sounds to be added to the ambient sound admitted through the IEMs. Such devices are also useful in quieter environments, where they serve as music- and sound-reproduction headphones that can reduce or block distracting environmental noise.
Because in-ear monitors and earplugs seal the ear canal, they create a small closed air volume with the eardrum (tympanic membrane) at one end. When the pressure in that volume is different from the pressure in the middle ear (which is more or less the same as ambient air pressure, mediated through the oral and nasal cavities and the Eustachian tube) it can cause discomfort during earplug insertion and removal, and “listening fatigue” during extended wear. However, adding a vent to equalize this pressure reduces the audio isolation and can disturb the desired acoustic balance.
Structures and methods to equalize ear-canal pressure without impairing audio balance may be of value in this field.